[1] Field of the Invention
The present invention relates to a pilot-operated three-position switching valve in which a spool is switched between three switching positions by a pilot fluid pressure and a return spring.
[2] Description of the Related Art
Pilot-operated three-position switching valves of this type have been known, as disclosed in, for example, Patent Literatures, Japanese Unexamined Patent Application Publication No. 59-190586 and Japanese Unexamined Patent Application Publication No. 11-182728. A known three-position switching valve is configured such that pistons are provided at both ends of a spool, which is slidably accommodated in a valve hole, a spring force of a return spring is applied the spool, the spool is moved in one direction and switched to a first switching position when a pilot fluid pressure is applied to one piston through one pilot valve, the spool is moved in the opposite direction and switched to a second switching position when a pilot fluid pressure is applied to the other piston through the other pilot valve, and the spool is switched to a neutral switching position by the return spring when both pilot valves are turned off and the pilot fluid pressure is removed from both pistons.
When the spool is switched to the first switching position, the switching valve goes into a first communication state in which some of a plurality of ports communicate with one another and the remaining ports are closed, when the spool is switched to the second switching position, the switching valve goes into a second communication state in which the communication-and-closure relationship of the plurality of ports is different from that in the first communication state, and when the spool is switched to the neutral switching position, the switching valve goes into a non-communication state in which all the ports are closed.
This is a so-called closed-center three-position switching valve, and its symbol is shown in FIG. 5. The three-position switching valve shown in FIG. 5 is a five-port switching valve.